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Nobel Lecture

Nobel Lecture, December 11, 1911

Radium and the New Concepts in Chemistry

Some 15 years ago the radiation of uranium
was discovered by Henri Becquerel1, and two years later the study of this
phenomenon was extended to other substances, first by me, and
then by Pierre Curie and myself2.
This study rapidly led us to the discovery of new elements, the
radiation of which, while being analogous with that of uranium,
was far more intense. All the elements emitting such radiation I
have termed radioactive, and the new property of matter
revealed in this emission has thus received the name
radioactivity. Thanks to this discovery of new, very
powerful radioactive substances, particularly radium, the study
of radioactivity progressed with marvellous rapidity: Discoveries
followed each other in rapid succession, and it was obvious that
a new science was in course of development. The Swedish Academy of
Sciences was kind enough to celebrate the birth of this
science by awarding the Nobel Prize for Physics to the first
workers in the field, Henri Becquerel, Pierre
Curie and Marie Curie (1903).

From that time onward numerous scientists
devoted themselves to the study of radioactivity. Allow me to
recall to you one of them who, by the certainty of his judgement,
and the boldness of his hypotheses and through the many
investigations carried out by him and his pupils, has succeeded
not only in increasing our knowledge but also in classifying it
with great clarity; he has provided a backbone for the new
science, in the form of a very precise theory admirably suited to
the study of the phenomena. I am happy to recall that Rutherford came to
Stockholm in 1908 to receive the Nobel Prize as a well-deserved
reward for his work.

Far from halting, the development of the
new science has constantly continued to follow an upward course.
And now, only 15 years after Becquerel's discovery, we are face
to face with a whole world of new phenomena belonging to a field
which, despite its close connexion with the fields of physics and
chemistry, is particularly well-defined. In this field the
importance of radium from, the viewpoint of general theories has
been decisive. The history of the discovery and the isolation of
this substance has furnished proof of my hypothesis that
radioactivity is an atomic property of matter and can provide a
means of seeking new elements. This hypothesis has led to
present-day theories of radioactivity, according to which we can
predict with certainty the existence of about 30 new elements
which we cannot generally either isolate or characterize by
chemical methods. We also assume that these elements undergo
atomic transformations, and the most direct proof in favour of
this theory is provided by the experimental fact of the formation
of the chemically defined element helium starting from the
chemically-defined element radium.

Viewing the subject from this angle, it can
be said that the task of isolating radium is the corner-stone of
the edifice of the science of radioactivity. Moreover, radium
remains the most useful and powerful tool in radioactivity
laboratories. I believe that it is because of these
considerations that the Swedish Academy of Sciences has done me
the very great honour of awarding me this year's Nobel Prize for
Chemistry.

It is therefore my task to present to you
radium in particular as a new chemical element, and to leave
aside the description of the many radioactive phenomena which
have already been described in the Nobel Lectures of H.
Becquerel, P. Curie and E. Rutherford.

Before broaching the subject of this
lecture, I should like to recall that the discoveries of radium
and of polonium were made by Pierre Curie in collaboration with
me. We are also indebted to Pierre Curie for basic research in
the field of radioactivity, which has been carried out either
alone, in collaboration with his pupils.

The chemical work aimed at isolating radium
in the state of the pure salt, and at characterizing it as a new
element, was carried out specially by me, but it is intimately
connected with our common work. I thus feel that I interpret
correctly the intention of the Academy of Sciences in assuming
that the award of this high distinction to me is motivated by
this common work and thus pays homage to the memory of Pierre
Curie.

I will remind you at the outset that one of
the most important properties of the radioactive elements is that
of ionizing the air in their vicinity (Becquerel). When a uranium
compound is placed on a metal plate A situated opposite another
plate B and a difference in potential is maintained between the
plates A and B, an electric current is set up between these
plates; this current can be measured with accuracy under suitable
conditions and will serve as a measure of the activity of the
substance. The conductivity imparted to the air can be ascribed
to ionization produced by the rays emitted by the uranium
compounds.

In 1897, using this method of measurement,
I undertook a study of the radiation of uranium compounds, and
soon extended this study to other substances, with the aim of
finding out whether radiation of this type occurs in other
elements. I found in this way that of the other elements known,
only the compounds of thorium behave like the compounds of
uranium.

I was struck by the fact that the activity
of uranium and thorium compounds appears to be an atomic
property of the element uranium and of the element thorium.
Chemical compounds and mixtures containing uranium and thorium
are active in direct proportion to the amount of these metals
contained in them. The activity is not destroyed by either
physical changes of state or chemical transformations.

I measured the activity of a number of
minerals; all of them that appear to be radioactive always
contain uranium or thorium. But an unexpected fact was noted:
certain minerals (pitchblende, chalcolite, autunite) had a
greater activity than might be expected on the basis of their
uranium or thorium content. Thus, certain pitchblendes containing
75% of uranium oxide are about four times as radioactive as this
oxide. Chalcolite (crystallized phosphate of copper and uranium)
is about twice as radioactive as uranium. This conflicted with
views which held that no mineral should be more radioactive than
metallic uranium. To explain this point I prepared synthetic
chalcolite from pure products, and obtained crystals, whose
activity was completely consistent with their uranium content;
this activity is about half that of uranium.

I then thought that the greater activity of
the natural minerals might be determined by the presence of a
small quantity of a highly-radioactive material, different from
uranium, thorium and the elements known at present. It also
occurred to me that if this was the case I might be able to
extract this substance from the mineral by the ordinary methods
of chemical analysis. Pierre Curie and I at once carried out this
research, hoping that the proportion of the new element might
reach several per cent. In reality the proportion of the
hypothetical element was far lower and it took several years to
show unequivocally that pitchblende contains at least one
highly-radioactive material which is a new element in the sense
that chemistry attaches to the term.

We were thus led to create a new method of
searching for new elements, a method based on radioactivity
considered as an atomic property of matter. Each chemical
separation is followed by a measurement of the activity of the
products obtained, and in this way it is possible to determine
how the active substance behaves from the chemical viewpoint.
This method has come into general application, and is similar in
some ways to spectral analysis. Because of the wide variety of
radiation emitted, the method could be perfected and extended, so
that it makes it possible, not only to discover radioactive
materials, but also to distinguish them from each other with
certainty.

It was also found in using the method being
considered, that it was in fact possible to concentrate the
activity by chemical methods. We found that pitchblende contains
at least two radioactive materials, one of which, accompanying
bismuth, has been given the name polonium, while the
other, paired with barium, has been called radium.

We were convinced that the materials which
we had discovered were new chemical elements. This conviction was
based solely on the atomic nature of radioactivity. But at first,
from the chemical viewpoint, it was as if our substances had
been, the one pure bismuth, and the other pure barium. It was
vital to show that the radioactive property was connected with
traces of elements that were neither bismuth nor barium. To do
that the hypothetical elements had to be isolated. In the case of
radium isolation was completely successful but required several
years of unremitting effort. Radium in the pure salt form is a
substance the manufacture of which has now been industrialized;
for no other new radioactive substance have such positive results
been obtained.

The radiferous minerals are being subjected
to very keen study because the presence of radium lends them
considerable value. They are identifiable either by the
electrometric method, or very simply by the impression they
produce on a photographic plate. The best radium mineral is the
pitchblende from St. Joachimsthal (Austria) which has for a long
time been processed to yield uranium salts. After extraction of
the latter, the mineral leaves a residue which contains radium
and polonium. We have normally used this residue as our raw
material.

The first treatment consists in extracting
the radiferous barium and the bismuth containing the polonium.
This treatment, which was first performed in the laboratory on
several kilograms of raw material (as many as 20 kg) had then to
be undertaken in a factory owing to the need to process thousands
of kilograms. Actually, we gradually learned from experience that
the radium is contained in the raw material in the proportion of
a few decigrams per ton. About 10 to 20 kg crude barium sulphate
containing radium are extracted from one ton of residue. The
activity of these sulphates is even then 30 to 60 times greater
than that of uranium. These sulphates are purified and converted
to chlorides. In the mixture of barium and radium chlorides the
radium is present only in the proportion of about 3 parts per
100,000. In the radium industry in France a much lower grade
mineral is most often used and the proportion indicated is far
lower still. To separate the radium from the barium I have used a
method of fractional crystallization of the chloride (the bromide
can also be used). The radium salt, less soluble than the barium
salt, becomes concentrated in the crystals. Fractionation is a
lengthy, methodical operation which gradually eliminates the
barium. To obtain a very pure salt I have had to perform several
thousands of crystallizations. The progress of the fractionation
is monitored by activity measurements.

A first proof that the element radium
existed was furnished by spectral analysis. The spectrum of a
chloride enriched by crystallization exhibited a new line which
Demarcay attributed to the new element. As the activity became
more concentrated, the new line increased in intensity and other
lines appeared while the barium spectrum became at the same time
less pronounced. When the purity is very high the barium spectrum
is scarcely visible.

I have repeatedly determined the average
atomic weight of the metal in the salt subjected to spectral
analysis. The method used was the one consisting in determining
the chlorine content in the form of silver chloride in a known
amount of the anhydrous chloride. I have found that this method
gives very good results even with quite small amounts of
substance (0.1 to 0.5 g), provided a very fast balance is used to
avoid the absorption of water by the alkaline-earth salt during
the weighings. The atomic weight increases with the enrichment of
the radium as indicated by the spectrum. The successive atomic
weights obtained were: 138; 146; 174; 225; 226.45. This last
value was determined in 1907 with 0.4 g of very pure radium salt.
The results of a number of determinations are, 226.62; 226.31;
226.42. These have been confirmed by more recent experiments.

The preparation of pure radium salts and
the determination of the atomic weight of radium have proved
positively that radium is a new element and have enabled a
definite position to be assigned to it. Radium is the higher
homologue of barium in the family of alkaline-earth metals; it
has been entered in Mendeleev's table in the corresponding
column, on the row containing uranium and thorium. The radium
spectrum is very precisely known. These very clear-cut results
for radium have convinced chemists and justified the
establishment of the new science of radioactive substances.

In chemical terms radium differs little
from barium; the salts of these two elements are isomorphic,
while those of radium are usually less soluble than the barium
salts. It is very interesting to note that strong radioactivity
of radium involves no chemical anomalies and that the chemical
properties are actually those which correspond to the position in
the Periodic System indicated by its atomic weight. The
radioactivity of radium in solid salts is ca. 5 million times
greater than that of an equal weight of uranium. Owing to this
activity its salts are spontaneously luminous. I also wish to
recall that radium gives rise to a continuous liberation of
energy which can be measured as heat, being about 118 calories
per gram of radium per hour.

Radium has been isolated in the metallic
state (M. Curie and A. Debierne, 1910). The method used consisted
in distilling under very pure hydrogen the amalgam of radium
formed by the electrolysis of a chloride solution using a mercury
cathode. One decigram only of salt was treated and consequently
considerable difficulties were involved. The metal obtained melts
at about 700°C, above which temperature it starts to
volatilize. Is it very unstable in the air and decomposes water
vigorously.

The radioactive properties of the metal are
exactly the ones that can be forecast on the assumption that the
radioactivity of the salts is an atomic property of the radium
which is unaffected by the state of combination. It was of real
importance to corroborate this point as misgivings had been
voiced by those to whom the atomic hypothesis of radioactivity
was still not evident.

Although radium has so far only been
obtained in very small amounts, it is nevertheless true to say,
in conclusion, that it is a perfectly defined and already
well-studied chemical element.

Unfortunately, the same cannot be stated
for polonium, for which nevertheless considerable effort has
already been spent. The stumbling block here is the fact that the
proportion of polonium in the mineral is about 5,000 times
smaller than that of radium.

Before theoretical evidence was available
from which to forecast this proportion, I had conducted several
extremely laborious operations to concentrate polonium and in
this way had secured products with very high activity without
being able to arrive at definite results as in the case of
radium. The difficulty is heightened by the fact that polonium
disintegrates spontaneously, disappearing by half in a period of
140 days. We now know that radium has not an infinite life
either, but the rate of disappearance is far less (it disappears
by half in 2,000 years). With our facilities we can scarcely hope
to determine the atomic weight of polonium because theory
foresees that a rich mineral can contain only a few hundredths of
a milligram per ton, but we can hope to observe its spectrum. The
operation of concentrating polonium, as I shall point out later,
is, moreover, a problem of great theoretical interest.

Recently, in collaboration with Debierne, I
undertook to treat several tons of residues from uranium mineral
with a view to preparing polonium. Initially conducted in the
factory, then in the laboratory, this treatment finally yielded a
few milligrams of substance about 50 times more active than an
equal weight of pure radium. In the spectrum of the substance
some new lines could be observed which appear attributable to
polonium and of which the most important has the wavelength
4170.5 Å. According to the atomic hypothesis of
radioactivity, the polonium spectrum should disappear at the same
time as the activity and this fact can be confirmed
experimentally,

I have so far considered radium and
polonium only as chemical substances. I have shown how the
fundamental hypothesis which states that radioactivity is an
atomic property of the substance has led to the discovery of new
chemical elements. I shall now describe how the scope of this
hypothesis has been greatly enlarged by the considerations and
experimental facts which resulted in establishing the theory of
atomic radioactive transformations.

The starting-point of this theory must be
sought in the considerations of the source of the energy involved
in the phenomena of radioactivity. This energy becomes manifest
as an emission of rays which produce thermal, electrical and
light phenomena. As the emission occurs spontaneously without any
known cause of excitation, various hypotheses have been advanced
to account for the liberation of energy. One of the hypotheses
put forward at the beginning of our research by Pierre Curie and
myself consisted in assuming that the radiation is an emission of
matter accompanied by a loss in weight of the active substances
and that the energy is taken from the substance itself whose
evolution is not yet completes and which undergoes an atomic
transformation. This hypothesis, which at first could only be
enunciated together with other equally valid theories, has
attained dominant importance and finally asserted itself in our
minds owing to a body of experimental evidence which
substantiated it. This evidence is essentially the following: A
series of radioactive phenomena exists in which radioactivity
appears to be tied up to matter in an imponderable quantity, the
radiation moreover not being permanent but disappearing more or
less rapidly with time. Such are polonium, radioactive emanations
and deposits of induced radioactivity.

It has been established moreover in certain
cases that the radioactivity observed increases with time. This
is what happens in the case of freshly prepared radium, of the
emanation freshly introduced into the measuring apparatus, of
thorium deprived of thorium X, etc.

A careful study of these phenomena has
shown that a very satisfactory general explanation can be given
by assuming that each time a decrease of radioactivity is
observed there is a destruction of radioactive matter, and that
each time an increase of activity is observed, there is a
production of radioactive matter. The radiations which disappear
and appear are, besides, of very varied nature and it is admitted
that every kind of rays determined can serve to characterize a
substance which is its source, and appears and disappears with
it.

As radioactivity is in addition a property
which is essentially atomic, the production or the destruction of
a distinct type of radioactivity corresponds to a production or a
destruction of atoms of a radioactive substance.

Finally, if it is supposed that radioactive
energy is a phenomenon which is borrowed from atomic
transformation, it can be deduced from this that every
radioactive substance undergoes such a transformation, even
though it appears to us to be invariable. Transformation in this
case is only very slow and this is what takes place in the case
of radium or uranium.

The theory I have just summarized is the
work of Rutherford and Soddy, which they have
called theory of atomic disintegration. By applying this
theory it can be concluded that a primary radioactive substance
such as radium undergoes a series of atomic transmutations by
virtue of which the atom of radium gives birth to a train of
atoms of smaller and smaller weights, since a stable state cannot
be attained as long as the atom formed is radioactive. Stability
can only be attained by inactive matter.

From this point of view one of the most
brilliant triumphs of the theory is the prediction that the gas
helium, always present in radioactive minerals, can represent one
of the end-products of the evolution of radium, and that it is in
the form of alpha rays that the helium atoms which are formed
when radium atoms distintegrate are discharged. Now, the
production of helium by radium has been proved by the experiments
of Ramsay and
Soddy, and it cannot now be contested that the perfectly defined
chemical element, radium, gives rise to the formation of another
equally defined element - helium. Moreover, the investigations
done by Rutherford and his students have proved that the alpha
particles emitted by radium with an electric charge are also to
be found in the form of helium gas in the space where they have
been recovered.

I must remark here that the bold
interpretation of the relationship existing between radium and
helium rests entirely upon the certitude that radium has the same
claim to be a chemical element as have all the other known
elements, and that there can be no question of regarding it to be
a molecular combination of helium with another element. This
shows how fundamental in these circumstances has been the work
carried out to prove the chemical individuality of radium, and it
can also be seen in what way the hypothesis of the atomic nature
of radioactivity and the theory of radioactive transformations
have led to the experimental discovery of a first
clearly-established example of atomic transmutation. This is a
fact the significance of which cannot escape anyone, and one
which incontestably marks an epoch from the point of view of
chemists.

Considerable work, guided by the theory of
radioactive transformations, has led to approximately 30 new
radioactive elements being envisaged, classified in 4 series
according to the primary substance: these series are uranium,
radium, thorium and actinium. The uranium and radium series can,
in fact, be combined, for it seems to be proved that radium is a
derivative of uranium. In the radium series the last known
radioactive body is polonium, the production of which by radium
is now a proven fact. It is likely that the actinium series is
related to that of radium.

We have seen that helium gas is one of the
products of radium distintegration. The helium atoms are detached
from those of radium and its derivatives during the course of the
transformation. It is supposed that after the departure of four
atoms of helium, the radium atom yields one atom of polonium; the
departure of a fifth helium atom determines the formation of an
inactive body with an atomic weight believed to be equal to 206
(20 units below that of radium). According to Rutherford this
final element is nothing more than lead, and this supposition is
now being subjected to experimental verification in my
laboratory. The production of helium from polonium has been
directly proved by Debierne.

The relatively large amount of polonium
prepared by Curie and Debierne has allowed an important study to
be undertaken. This consists in counting a large number of alpha
particles emitted by polonium and in collecting and measuring the
corresponding volume of helium. Since each particle is a helium
atom, the number of helium atoms is thus found which occupy a
given volume and have a given weight. It can therefore allow us
to deduce, in a general way, the number of molecules in a
grammolecule. This number, known as Avogadro's constant, is of
great importance. Experiments conducted on polonium have supplied
a first value for this number, which is in good agreement with
the values obtained by other methods. The enumeration of alpha
particles is done by an electrometric method due to Rutherford;
this method has been brought to perfection by means of a
photographic recording apparatus.

Recent investigations have shown that
potassium and rubidium emit a very feeble radiation, similar to
the beta radiation of uranium and radium. We do not yet know
whether we should regard these substances as true radioactive
bodies, i.e. bodies in the process of transformation.

To conclude I should like to emphasize the
nature of the new chemistry of radioactive bodies. Tons of
material have to be treated in order to extract radium from the
ore. The quantities of radium available in a laboratory are of
the order of one milligram, or of a gram at the very most, this
substance being worth 400,000 francs per gram. Very often
material has been handled in which the presence of radium could
not be detected by the balance, nor even by the spectroscope. And
yet we have methods of measuring so perfect and so sensitive that
we are able to know very exactly the small quantities of radium
we are using. Radioactive analysis by electrometric methods
allows us to calculate to within 1% a thousandth of a milligram
of radium, and to detect the presence of 10-10 grams
of radium diluted in a few grams of material. This method is the
only one which could have led to the discovery of radium in view
of the dilution of this substance in the ore. The sensitivity of
the methods is still more striking in the case of radium
emanation, which can be detected when the quantity present
amounts, for example, to only 10-10 mm3. As the specific activity of a substance is,
in the case of analogous radiations, approximately in inverse
proportion to the average life, the result is that if the average
life is very brief, the radioactive reaction can attain an
unprecedented sensitivity. We are also accustomed to deal
currently in the laboratory with substances the presence of which
is only shown to us by their radioactive properties but which
nevertheless we can determine, dissolve, reprecipitate from their
solutions and deposit electrolytically. This means that we have
here an entirely separate kind of chemistry for which the current
tool we use is the electrometer, not the balance, and which we
might well call the chemistry of the imponderable.